The present invention relates to a light-amplifying optical fiber applicable to optical fiber amplifiers, optical fiber laser oscillators,and the like; and a method of making the same.
A light-amplifying optical fiber is an optical waveguide in which a rare-earth element such as Er is added into a core region. It has been known that, when an optical fiber doped with a rare-earth element is supplied with light having a wavelength capable, of exiting the rare-earth element, then a population inversion is formed therein, whereby a stimulated emission at a wavelength corresponding to the wavelength of excitation light occurs. Therefore, light-amplifying optical fibers are widely utilized in optical fiber amplifiers for amplifying signal light having a wavelength coinciding with the wavelength of the stimulated emission light and in optical fiber laser oscillators for outputting laser oscillation light having a wavelength coinciding with the wavelength of the stimulated emission light. Optical fiber amplifiers and optical fiber laser oscillators are desired to have a high and flat gain characteristic or oscillation characteristic in a wider wavelength band. Consequently, the light-amplifying optical fibers employed in the optical fiber amplifiers, optical fiber laser oscillators, and the like have been under study and development from such a viewpoint.
For example, Japanese Patent Application Laid-Open No. HEI 1-145881 discloses a light-amplifying optical fiber in which a part of a core region including its optical axis center is doped with Er element. The light-amplifying optical fiber according to this first conventional example attains a higher gain by approximating the intensity distribution of signal light or laser oscillation light and the distribution of Er element to each other. On the other hand, Japanese Patent Application Laid-Open No. HEI 5-283789 discloses a light-amplifying optical fiber in which Er element is added to the whole core region, whereas Al2O3 is added into the core region. Due to such a structure in which Er element and Al2O3, are added into the same region, the light-amplifying optical fiber according to this second conventional example attains a wider and flatter amplification wavelength band.
Having studied-conventional techniques such as those mentioned above, the inventors have found problems as follows. When an optical fiber in which at least a part of a core region including its optical axis center is doped with both of Er element and Al2O3 is employed, a higher gain in optical fiber amplifiers and the like, and a wider amplification wavelength band and flatter, gain in their amplification wavelength band can be expected. However, since Er element and Al2O3 diffuse due to the heating and softening of the optical fiber preform in the process of making the optical fiber employed in optical fiber amplifiers and the like, it is difficult for the added Er element and Al2O3 to be confined only in a part of the core region of the optical fiber in practice. If Er element also exists in an area with a low Al2O3 content due to the diffusion mentioned above, the amount of Er element binding to the main ingredient, SiO2, without binding to Al2O3 will increase. As a result, optical fiber amplifiers and the like employing conventional light-amplifying optical fibers such as those mentioned above may not fully achieve a wider band and flatter gain in their amplification wavelength bands.
If the doped amount of Al2O3 is enhanced in order to overcome a problem caused by the diffusion of Er element as mentioned above, on the other hand, then crystals of Al2O3 are more likely to be formed. Therefore, there is a limit to the doped amount of Al2O3, which inevitably restricts the widening of band and flattening of gain in amplification wavelength bands in optical fiber amplifiers and the like.
While in the case of a light-amplifying optical fiber whose core region is wholly doped with Er element and Al2O3 it is comparatively unnecessary to take account of the diffusion of Er element and the like upon the heating and softening of the optical fiber preform during the manufacturing process. In the case of such a light-amplifying optical fiber, in which the doped amount of Al2O3 in the core region can be easily increase, it appears to be more likely to achieve a wider band and flatter gain in amplification wavelength bands in optical fiber amplifiers and the like as compared with the above-mentioned light-amplifying optical fiber in which only a part of its core region is doped with Er element or the like. Even in this case, however, Er element and the like cannot be kept from diffusing upon the heating and softening of the optical fiber preform in its making process, whereby the possibility of increasing Er element binding to the main material, SiO2, without binding to Al2O3 cannot be denied. Hence, there is still a limit to the widening of band and flattening of gain in amplification wavelength bands in optical fiber amplifiers and the like employing an optical fiber whose whole core region is doped with both of Er element and Al2O3.
In order to overcome the above-mentioned problems, it is an object of the present invention to provide a light-amplifying optical fiber comprising a structure capable of achieving a higher and flatter gain characteristic or oscillation characteristic in a wider wavelength band; and a method of making the same.
The light-amplifying optical fiber according to the present invention is an optical component applicable to optical fiber amplifiers, optical fiber laser oscillators, and the like; and comprises a core region extending along a predetermined axis, and a cladding region, provided on the outer periphery of the core region, having a refractive index lower than that of the core region.
In particular, the light-amplifying optical fiber according to the present invention comprises a first doped area, with an outer diameter a, extending along the predetermined axis; and a second doped area, with an outer diameter b ( greater than a), containing the first doped area. The first doped area is a glass region doped with at least a rare-earth element, for example, such as one of Er, Nd, Tm, Yb, and Pr, within the light-amplifying optical fiber. The second doped area is a glass region doped with at least an oxide of an element having a valence different from that of a cation constituting a main material, SiO2, of the light-amplifying optical fiber, for example, such as one of Al2O3, P2O5, Y2O3, and B2O3, containing the first doped area.
In this case, even if the rare-earth element and the oxide such as Al2O3 diffuse upon the heating and softening of the optical fiber preform in the process of making the light-amplifying optical fiber, the rare-earth element diffused from the first doped area will substantially reside within the second doped area and thus will be more likely to bind to the oxide than to the main material of the light-amplifying optical fiber. Hence, by providing a structure in which the diffusing rare-earth element and the oxide such as Al2O3 are likely to bind together, the light-amplifying optical fiber according to the present invention attains a higher and flatter gain characteristic or oscillation characteristic in a wider wavelength band.
Preferably, the doped amount of the oxide such as Al2O3 added to the second doped area is substantially uniform in a diametric direction of the light-amplifying optical fiber orthogonal to an optical axis thereof.
In the light-amplifying optical fiber according to the present invention, while the second doped area contains the first doped area, the first and second doped areas and the core region have various modes of relationships therebetween.
For example, they may be configured such that the first doped area is formed as a part of the core region, whereas the second doped area containing the first doped area coincides with the core region (first embodiment). In this first embodiment, while the outer diameter of the first doped area is smaller than that of the core region, the outer diameter of the second doped area coincides with that of the core region. Also, each of the first and second doped areas may constitute a part of the chore region (second embodiment). In this second embodiment, the outer diameter of each of the first and second doped areas is smaller than that of the core region. In a third embodiment, the first doped area constitutes the core region, whereas the second doped area constitutes a part or the whole of the cladding region. In the third embodiment, the outer diameter of the first doped area coincides with that of the core region. While the structural relationship between the first and second doped areas in a fourth embodiment is similar to that in the first embodiment, the structure of the core region itself in the light-amplifying optical fiber differs from that in the first embodiment. Namely, it is not always necessary for the respective parts of the core region constituting the first and second doped areas to have refractive indices identical to each other.
Further, the core region in the light-amplifying optical fiber according to the invention is doped with at least, one of GeO2 and a halogen element. When doped with GeO2 or Cl element, which is a halogen element, the core region can raise its refractive index on the other hand, F element, which is a halogen element, can be utilized as a refractive index lowering agent in the cladding region. When a halogen element is added together with GeO2, the degree of freedom in designing the refractive index profile enhances. Also, even in the case where the same form of refractive index profile is to be obtained, the doped amount of GeO2 can be made relatively smaller, whereby it is preferable for lowering transmission loss.
In the light-amplifying optical fiber according to the present invention, the ratio (a/b) of the outer diameter a of the first doped area to the outer diameter b of the second doped area is preferably 0.1 or more but 0.9 or less. When the outer diameter of lathe second doped area is thus set greater than that of the first doped area, the binding between the rare-earth element and the main material of the light-amplifying optical fiber, SiO2, is suppressed, whereby a sufficient gain is obtained.
In the method of making a light-amplifying optical fiber according to the present invention, a first area (corresponding to an outer part of the second doped area) containing an oxide of an element having a valence different from that of a cation constituting a main material of a glass pipe is initially formed one an inner wall of the glass pipe (first step). Subsequently, on an inner wall of the first area formed by the first step, a second area (corresponding to the first doped area and an inner part of the second doped area) doped with a rare-earth element and the oxide is formed (second step). The glass pipe thus formed is solidified (third step), whereby an optical fiber preform is obtained. This optical fiber preform is drawn (fourth step), so as to yield the light-amplifying optical fiber configured such that the first doped area doped with at least a rare-earth element is contained in the second doped area doped with an oxide such as Al2O3. Here, each of the first and second areas respectively formed in the first and second steps constitutes a glass region to become the core region of the light-amplifying optical fiber.